The present invention relates to a filtering device for a spinning head and a spinning head for the manufacture of plastic threads. Plastic melt flows in a housing through a filter element with filtering material before being discharged under pressure through the spinnerets. The filter element has a filtering material subjected through a distribution device to an axial pressing force dependent upon the strength or intensity of the pressure of the melt. For closing of sealing gaps, the axial pressing force works on the sealing edges of the filter element.
With melt-spinning, the polymer melt must be subjected to a super-fine filtration by a sand filter, a metal powder filter or filter cartridges immediately before passage through the spinneret plate. EP 0 658 638 B1 discloses conducting the polymer melt through a plurality of metal gauze filter cartridges arranged parallel to one another. Disadvantageously, uniform flow through all of the filter cartridges is practically impossible, with the result that certain individual filter cartridges become polluted before the others. Entire sets of filter cartridges must be cleaned or exchanged following short operation times.
With a spinning head of the type shown in DE 42 25 341 A1, one single annular filter element is used and surrounds or encloses a mixed operation device in the interior of the housing. This arrangement serves to bring together the filtered melt flow which has been homogeneously and thoroughly mixed coming out of the filter. However, the conventional filter element is not sufficiently pressure-stable and the elasticity of the stretched out filter element does not suffice to close over its sealing edges to form a complete sealing of those potential passage points present in the form of sealing gaps. In order to overcome this drawback, presently the annular filter element has been replaced by plate filter packs. However, disruptive energy flow behavior arises as a result of the reorientation of the melt flow within the filter element with its plate filter disks when such a reorientation is thus required. The reorientation hinders the relevant filtering process for the plastic melt. Also, such a solution is costly in realization.
DE 42 27 114 discloses a spinning head whereby the stability of the annular filter element is increased on the basis of radial support obtained by a distribution device. However, its filter element allows only a very uneven flowthrough.
U.S. Pat. Nos. 4,661,249 and 2,881,474 disclose filter elements each incorporating a support pipe which has built-in fluid outlets in the walls surrounded or enclosed by a filtering material. The filter element in this case is stretched tightly around the ends in the housing parts of the filter device. At the ends, the sealing attained exists only within the range of predeterminable sealing values. Because of the static stretching, settling within the seals and a loss of seal effectiveness results.
Objects of the present invention are to provide an improved filter device for a spinning head, which allows energy-saving filtering operation, which is pressure-stable, which guarantees a good duration of the seal and which is of low cost in its realization.
Other objects of the present invention are to provide a spinning head in which a particularly homogenous plastic melt for the production of a multi-strand synthetic thread appears at the point of discharge.
The foregoing objects are obtained where the filter element, formed as an exchangeable structural unit, includes a support pipe with built-in fluid outlets in the wall. The support pipe is surrounded or enclosed by the filtering material. The filter material projects out axially at one end and/or is displaced axially rearward at least at one end, with its sealing edge over the support pipe. A pressure-stable structural unit is thus formed, which is operationally secure even when working with high melt pressures. Since the plastic melt flows in the direction of and uniformly over the entire length of the filter element in one direction, the flowthrough is opposed or contrasted without waste of energy. No artificial resistance is built up counter to the melt flowthrough. Through the projecting end or the rearwardly aligned end of the filtering material, which forms the relevant sealing edge, with the given elasticity behavior of the filter element, secure sealing of the possible passage points in the form of potential fissures in the seal is guaranteed. Since such a filter element can be manufactured of standard structural parts, especially of traditional metal gauze filtering materials, the filter element is of low cost to manufacture.
Since at least at one end, a flat sealing edge can be present in the case of the filter element, the overall rigidity of the filter element is enhanced. The closing of the sealing gap occurs under the force of the pressure being exerted. Dependent upon the pressing force, a pro-actively working sealing arrangement is thus realized for the filtering device with the spinning head, which seals progressively more tightly with application of progressively increasing pressure.
The spinning head according to the present invention is characterized especially in that the plastic melt is guided uniformly in its movement from the inlet all the way to the discharge and without remaining for different lengths of time in different areas. The output chamber formed between the melt passage support pipe and the distribution head guarantees uniform flowthrough through the filter element and unhindered carrying and removal of the filtered plastic melt to the discharge point. A high degree of pressure stability of the filter element is attained because of the support pipe.
In order to obtain a favorable flow passage, it is especially advantageous to widen the open flowthrough diameter of the output chamber by conical tapering of the distribution head in the direction of the discharge.
In order to improve the inlet passage of the plastic melt from the side into the filter element, a bottom of the pressure plate can be phased in on the displacement device between the pressure plate and the distributing head. The pressure plate operates as a sealing surface with the sealing edge of the filter element. Flow accumulation angles which are disadvantageous to the operation are thus avoided.
For this purpose, the bottom of the pressure plate is preferably inclined at an angle of from 0xc2x0 to 40xc2x0 in the direction of discharge of the plastic melt in the spinning head.
For the melt at the ends of the filter element, and especially at the end of the filter element turned facing or adjacent the pressure plate, to flow smoothly and without remarkable hindrance into the filtering material, a cylindrical filter cover is connected to both the support pipe and the filtering material. The seal surfaces of the filter element are constructed at the end of the cover. Especially advantageously, the filtering material at the end of the filter element turned toward or adjacent the pressure plate project out axially with its sealing edge over the support pipe or else be angled rearward, so that accumulation angles no longer occur.
The elasticity of the filter cover surprisingly is not limited by the radial stability of the support pipe. Through the fluid passages in the support pipe, the filter cover has an axial elasticity which under the axial pressing force pro-actively supports or assists with the closing of the sealing gaps around the sealing edges of the filter element. The elasticity of the filter element can thus be enhanced and increased, while the filter element does not have at its disposal traditional end caps of correspondingly rigid material. Rather, the elasticity is attained solely by consolidation of the filtering material and corresponding fusing or welding of the ends.
Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses preferred embodiments of the present invention.